Dave Koehler has been a drag racer, machinist and fuel-flow
specialist for over 30 years. In response to all the Internet
handwringing concerning recent disqualifications at the March
Meet, we asked him to give our readers a quick tutorial on measuring fuel pump output in GPMs.


Since the word “certified” has been thrown around the net by non-bench owners, I will ask this: What certification? By what industry? Using what fluid?

Answer: There isn't one and it's a question that can't be answered by anyone in our unique little part of the world. As you read on you will find other reasons why this is tough to implement. You can bet the farm that if such a certification/blessing were possible that little 50-60 buck test (which is too cheap, by the way) would instantly become a $300 test. Be careful what you wish for.


That can be done by each shop for each turbine (used in the bench) that has a K factor. The K factor that comes with the turbines is somewhat generic and I think an averaged number that comes with every one of them. The manufacturers do not test each turbine before they go out the door or the turbines would be even more overpriced than they already are. They also do not calibrate them for anything other than water. As an example, I have had two identical turbines that don’t deliver the same result despite the fact they both claim the same K factor. Begin to see part of the problem?

Here's the thing. If you have a turbine that has an accuracy rating of +/- 1%, that sounds pretty good, right? But that is really a 2% spread.

So, if you have 21 gallon per minute (gpm) and the planets align then it could deliver an number anywhere between 20.79 and 21.21 gpm. If you have a nicer (pricier) turbine rated at +/- .25% then you could see between 20.94 and 21.05 gpm. Both of the high numbers shown currently would get you thrown under the bus.

Now throw in liquid density, temperature and the way a data logger or gauge reads out and you have a lot more variables. What number do you believe?


These turbines (with few exceptions) come with instructions that there must be X length of pipe before and after the sensor for the greatest accuracy. Has anyone seen a sensor installed on a race car following this recommendation? I think not. FYI, the recommendations are often longer than the engine!

Knowing that, I just install them where they do the most good while testing. Much like on the car. If it throws it off, so be it. It will be off on the race car too.

I also came to the conclusion that turbine calibrating assumes a nice steady flow and pressure, but there is nothing steady or nice about fuel injection on a race engine. Therefore I tend to take things I see sometimes with a big slug of salt.